This invention relates to a process for producing short fibrillar cellulose ester material (termed fibrets) for use as a replacement for clay and titanium dioxide in paper opacification. The process is also applicable to the production of other short length cellulose ester fibrillar structures such as flake, rough surface staple fibers and pulp suitable for the formation of a cellulose ester paper sheet for use in the production of tobacco smoke and other filters. Cellulose ester fibrets can be used to replace the normal high opacity paper fillers, such as clay and titanium dioxide, with a high retention of the filler in the paper making process and without significant deterioration in paper strength, making it possible to produce lower weight basis paper of substantially equal opacity and strength.
High opacity fillers make paper opaque by refracting and reflecting light rays striking the paper sheet. The light rays are refracted when there is a difference in the refractive indices between air and the paper filler. A solid material with a high index of refraction, such as titanium dioxide, can thus increase the opacity of paper when in admixture with the base cellulose paper fibers. However, the very small particle size and granular shape of such fillers leads to a reduction in sheet strength proportional to the amount included. Alternatively, a material of lower refractive index can provide an equal opacification effect by having an extensive void structure characterized by a high specific surface area. Light reflected and refracted in passing into and through the numerous voids is thoroughly scattered and imparts an opaque appearance to the material.
The fibrillar nature of the filler of this invention permits an intimate incorporation of the filler with the base cellulose paper fibers and a frictional or entanglement bonding therewith. The sheet strength is thus maintained at a higher level than that found for equal amounts of granular fillers.
The production of one type of cellulose ester fibrous filler is described in U.S. Pat. Nos. 3,342,991 and 3,441,473. This process consists of air spraying a dilute dope of the cellulose ester into the atmosphere and then into a quench bath of water. Similar cellulose ester fibers are disclosed in U.S. Pat. No. 2,988,469. In that process, cellulose acetate dope is extruded into a high velocity, unidirectional, free flowing jet stream to produced fibers without the formation of shot.
Alternatively, high surface area fibrillar materials obtained from wholly synthetic polymers are described in U.S. Pat. Nos. 2,988,782 and 2,999,788. These are produced by processes characterized by a rapid precipitation of a polymer in a region of high shear. Conversely, the process of the present invention is characterized by a rapid diffusional precipitation of a dissolved cellulose ester polymer in a region of draw sufficient to rupture the partially formed filaments. It is further characterized by a shear rate substantially lower than that of the prior art.
The trend in the paper industry toward light weight papers has necessitated finding a means for maintaining, in light weight sheets, optical properties normally found in heavier weight papers. The utilization of such conventional fillers as clay and titanium dioxide, in greater amounts to achieve the desired optical properties results in a deterioration of the strength of such light weight papers, thus severely limiting their usefulness. The use of cellulose ester fibrets produced by the process of this invention, provides a vehicle for obtaining high opacity light weight papers, with 90 percent or more retention of the fibrets in the paper making process and without serious deterioration of paper strength.